Porphyrins in coal

Abstract

The background to recent developments in investigations on coal porphyrins is reviewed. Essentially all the work of the past ten years has been with lignites and coals of the humic series. The concentrations of porphyrins and metalloporphyrins are low (0-ca. 10 μg g −1), which makes for difficulties in isolation and analysis. The preferred methods of isolation and analysis are summarised. The coal porphyrins differ from the porphyrins from crude oil, oil shale and related deposits in a variety of ways which are discussed. The major differences are the predominance of the etio series over the cycloetio series (DPEP series), the presence of Fe and Ga porphyrins (rather than Ni and VO porphyrins), and the presence of mesoporphyrin IX in lignite. Recent results tend to support Treibs' original idea (1935) that, for the coal porphyrins, both chlorophyll and iron porphyrin (haem) sources are important. The application of a weighted mean molecular mass of the porphyrins present as a biological marker in determining coal rank (Porphyrin Index of Coalification) is illustrated. Current activity is directed to the isolation and identification of individual iron complexes, and here a combination of thin layer chromatography and paramagnetically shifted 1H-NMR spectroscopy (of dicyanoferrihaems), together with direct comparison with authentic haems especially synthesised for the purpose, has been rewarding.